In most countries there are regulations in force which place certain maximum limits on the power of electromagnetic radiation directed towards humans. These limits vary from country to country and will most probably be tightened in the future.
On the other hand, as the transmission speed in mobile communication means systems increases, the amount of energy radiated by mobile communication means also increases. For example, in networks based upon TDMA technology, data transmission speeds may in principle be increased by using several successive time slots in the same TDMA frame, in which case the mean transmitted power increases. The use of several successive time slots in the same frame is possible in the Universal Mobile Telecommunication System (UMTS) under development and in new Global System for Mobile communications (GSM) standards which are under development. In CMDA systems the transmitted power has to be raised in line with the data transmission speed if one wishes to keep the bit error rate constant. In both basic techniques the amount of energy radiated by the antenna increases as the data transmission speed increases, in which case the permitted limits may at some stage be exceeded.
The radiation power caused by a particular radio set and directed towards the user is typically measured on the basis of the least favourable operating situation, in the case of a cordless telephone with the antenna almost pressed against the user's head. If a radio set may be used in many ways demanding differing transmitted power, for example for voice transmission and for high-speed data transfer, all functions of the radio set have to adjust to the limits which follow from the least favourable operating situation.
For example in the current GSM system, the maximum power of a mobile communication means which is used during one time slot is 2 W, which means that the mean transmitted power is approximately 250 mW. According to new GSM standards which are being developed, a mobile communication means may use for the high-speed data transfer as many as all eight time slots in one frame, in which case the mean transmitted power is 2 W. This causes problems if the 250 mW which is satisfactory for voice calls has been set as the upper limit of radio power directed towards the user's head. In this situation the mobile communication means may not use maximum power levels, but must use instantaneous transmitted power lower than in a voice call, so that the limit of mean transmitted power is not exceeded. This again causes audibility problems, since the base stations in the mobile communication means network are designed to cope with the needs of a voice link.